Polypropylene (PP) fibers and filaments are items of commerce and have been used in making products such as ropes, non-woven fabrics, and woven fabrics.
U.S. Pat. No. 4,578,414 discloses additives for making olefin polymer fibers water-wettable, including blends of polyethylene (PE) and polypropylene (PP).
U.S. Pat. No. 4,518,744 discloses melt-spinning of certain polymers and blends of polymers, including polypropylene (PP). Japanese Kokai 56-159339 and 56-59340 disclose fibers of mixtures of polyester with minor amounts of polypropylene.
Convenient references relating to fibers and filaments, including those of man-made thermoplastics, and incorporated herein by reference, are, for example:
(a) Encyclopedia of Polymer Science and Technology, Interscience, New York, Vol. 6 (1967), pp. 505-555 and Vol. 9 (1968), pp. 403-440; PA1 (b) Man-Made Fiber and Textile Dictionary, published by Celanese Corporation; PA1 (c) Fundamentals of Fibre Formation--The Science of Fibre Spinning and Drawing, by Andrzij Ziabicki published by John Wiley & Sons, London/New York, 1976; PA1 (d) Man-Made Fibres, by R. W. Moncrieff, published by John Wiley & Sons, London/New York, 1975; PA1 (e) Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 16 for "Olefin Fibers", published by John Wiley & Sons, New York, 1981, 3rd Edition.
In conformity with commonly accepted vernacular or jargon of the fiber and filament industry, the following definitions apply to the terms used in this disclosure:
A "monofilament" (a.k.a. monofil) refers to an individual strand of denier greater than 15, usually greater than 30;
A "fine denier fiber or filament" refers to a strand of denier less than about 15;
A "multi-filament" (a.k.a. multifil) refers to simultaneously formed fine denier filaments spun as a bundle of fibers, generally containing at least 3, preferably at least about 15-100 fibers and can be several hundred or several thousand;
"Staple fibers" refer to fine denier strands which have been formed at, or cut to, staple lengths of generally about 1 to about 8 inches;
An "extruded strand" refers to an extrudate formed by passing polymer through a forming-orifice, such as a die.
A "fibril" refers to a superfine discrete filament embedded in a more or less continuous matrix.
Whereas it is known that virtually any thermoplastic polymer can be extruded as a coarse strand or monofilament, many of these, such as polyethylene and some ethylene copolymers, have not generally been found to be suitable for the making of fine denier fibers or multi-filaments. Practitioners are aware that it is easier to make a coarse monofilament yarn of 15 denier than to make a multi-filament yarn of 15 denier. It is also recognized that the mechanical and thermal conditions experienced by a bundle of filaments, whether in spinning staple fibers or in multi-filaments yarns, are very different to those in spinning monofilaments. The fact that a given man-made polymer can be extruded as a monofilament, does not necessarily herald its use in fine denier or multi-filament spinning. Whereas an extruded monofilament which has been cooled can usually be cold-drawn (stretched) to a finer denier size, even if it does not have sufficient melt-strength to be melt-drawn without breaking, it is apparent that a polymer needs to have an appreciable melt-strength to be hot-drawn to fine denier sizes.
Low density polyethylene (LDPE) is prepared by polymerizing ethylene using a free-radical initiator, e.g. peroxide, at elevated pressures and temperatures, having densities in the range, generally, of about 0.910-0.935 gms/cc. The LDPE, sometimes called "I.C.I.-type" polyethylene is a branched (i.e. non-linear) polymer, due to the presence of short-chains of polymerized ethylene units pendent from the main polymer backbone. Some of the older art refers to these as high pressure polyethylene (HPPE).
High density polyethylene (HDPE) is prepared using a coordination catalyst, such as a "Ziegler-type" or "Natta-type" or a "Phillips-type" chromium oxide compound. These have densities generally in the range of about 0.94 to about 0.98 gms/cc and are called "linear" polymers due to the substantial absence of short polymer chains pendent from the main polymer backbone.
Linear low density polyethylene (LLDPE) is prepared by copolymerizing ethylene with at least one alpha-olefin alkylene of C.sub.3 -C.sub.12, especially at least one of C.sub.4 -C.sub.8, using a coordination catalyst such as is used in making HDPE. These LLDPE are "linear", but with alkyl groups of the alpha-olefin pendent from the polymer chain. These pendent alkyl groups cause the density to be in about the same density range (0.88-0.94 gms/cc) as the LDPE; thus the name "linear low density polyethylene" or LLDPE is used in the industry in referring to these linear low density copolymers of ethylene.
Polypropylene (PP) is known to exist as atactic (largely amorphous), syndiotactic (largely crystalline), and isotactic (also largely crystalline), some of which can be processed into fine denier fibers. It is preferable, in the present invention, to use the largely crystalline types of PP suitable for spinning fine denier fibers, sometimes referred to as "CR", or constant rheology, grades.
U.S. Pat. Nos. 4,181,762, 4,258,097, and 4,356,220 contain information about olefin polymer fibers, some of which are monofilaments.
U.S. Pat. No. 4,076,698 discloses methods of producing LLDPE and discloses extrusion of a monofilament.
It has now been found, unexpectedly, that improvements are made in polypropylene fibers if the polypropylene is first blended with about 20% to about 45% by wt. of a polyethylene, especially a linear low density ethylene copolymer (LLDPE) containing, generally, about 3% to about 20% of at least one alpha-olefin alkylene of 3-12 carbon atoms. It was also found that certain polyethylenes (more specifically LLDPE's) can be blended in a molten state with polypropylene in all proportions and then melt spun into fine denier fibers, some of which offer improved properties over polyethylene and polypropylene alone.